The classic guitar includes a sound box which is covered by a soundboard and has an elongated neck which carries a finger board and tensioning device at its free end for adjusting the frequency or pitch of the strings. A bridge portion is provided on the soundboard to permit mounting of a saddle over which the guitar strings are trained, and the ends of the guitar strings opposite to the tensioning device are suitably anchored by an anchor bar affixed to the soundboard. In the conventional acoustic guitar, sounds may be amplified by incorporating an electrical pick-up or transducer into the soundboard. Generally, in the past this has been done by adhering a wafer-type transducer under or adjacent to the bridge either in the original construction of the instrument or as a replacement or accessory, and is typically referred to as a body or contact transducer; or an alternate approach has been to place a number of individual transducers under the saddle in a bridge especially constructed for that purpose during original construction.
The generation of electrical signals by an electromechanical transducer is a well understood phenomenon. For instance, a piezoelectric ceramic transducer is a polarized synthetic crystal which emits a small voltage across the electrodes when it is subjected to stress, such as, the stress produced by vibration of the instrument strings. The piezoelectric crystal is so designed as to have a lattice oriented in such a way as to be polarized, and the stress imparted to the lattice alters its electrical potential so as to emit a corresponding voltage across the electrodes. Since the lattice is so oriented, the crystal is more sensitive to string vibrations which create stress in the direction of polarization.
Particular problems have been associated with electronic amplification of acoustical instruments. When a transducer is attached to the body of the guitar, such as, the sound box or sound board, the tonal qualities of the instrument are substantially retained. However, the disadvantage of this approach to electronic amplification is that there is both reduced sensitivity of the pickup unit and amplification of unwanted sounds resulting from taps or thumps on the body of the instrument as well as string squeak, and such unit is especially susceptible to feedback making it very difficult to achieve enough sound level or loudness to play to large audiences. This is due at least in part to the top itself acting as a receiver of sympathetic resonances from the loudspeakers. Where vintage instruments are concerned, the attachment of a transducer device may be troublesome since it may require the drilling or cutting of mounting holes in the instrument or other alterations thereto. The present invention avoids these problems by providing a sensitive piezoelectric transducer formed as part of an interchangeable saddle member which may be directly mounted in the original saddle slot of the bridge of the instrument. This invention therefore combines the best qualities of both the contact transducer which is attached directly to the sound board and the string transducer which underlies the instrument strings.
Various approaches have been taken in the installation of piezoelectric transducers to meet the problems associated with electronic amplification of an acoustical or stringed instrument, but generally can be characterized as requiring either that the transducer be mounted under the bridge or in a specially constructed bridge to receive the elements, making it necessary to purchase the instrument with the device already installed at the factory. For example, U.S. Pat. No. 3,712,951 issued Jan. 23, 1973 to Rickard discloses a special bridge assembly which has a number of individual piezoelectric transducers corresponding to the number of strings of the instrument. By utilizing individual transducers, cross-coupling of the strings is minimized, and the sympathetic resonance of the entire set of strings as well as the soundbox is thereby limited, and may alter the fullness of tone generated by the instrument. U.S. Pat. Nos. 3,396,284 and 3,530,228 to Scherer also disclose individual transducers corresponding to individual strings of the instrument wherein the crystals are isolated from one another and employ acoustical damping in an attempt to damp out undesirable highs, thumps and finger noise. Units of the type described above are generally acknowledged in the trade as the heretofore most versatile and popular units. This is due primarily to the cleanliness of tone resulting from the individual elements, lack of cross-coupling and relative absence of feedback. A drawback to this approach is that the quality or fidelity of the sound must be sacrificed and has posed certain problems heretofore in mounting and adaptation to existing guitars.
U.S. Pat. No. 3,507,972 utilizes an elastic material between the saddle and bridge components of the assembly to acoustically damp the saddle from the soundboard of the instrument. The elastic material places the transducer under compression so as to increase the output for a given amplitude of vibratory motion; however the elastic material absorbs the vibratory energy of the string which would reduce sustain times for a given note. Further, the amplified tone would have a quick rise time due to rapid absorption of string energy which would not be compatible with the tones expected to be produced by a non-amplified acoustical instrument.
U.S. Pat. No. 3,291,887 to Carman et al discloses a transducer element placed in an open gap of a saddle so that relative movement between opposing walls of the gap supporting the piezoelectric crystal cause it to be compressed to produce a modulated electric signal that may be amplified. Other patents such as U.S. Pat. No. 3,325,580 to Barcus et al and U.S. Pat. No. 4,147,084 to Underwood disclose violin bridges wherein the wings of the bridge are split with piezoelectric crystals being placed in the split wing so that it is sensed as the bridge vibrates. These devices naturally suppress the acoustic sound due to the crystal impeding the bridge wing motion.
A three-dimensional transducer is disclosed in U.S. Pat. No. 3,624,264 to Lazarus which is provided with three transducer crystals mounted on mutually perpendicular interior walls of the transducer assembly. This assembly is then filled with silicone rubber and attached directly to the soundboard of the instrument. While this transducer does not interfere with the acoustic circuit of the instrument, as is often the case with many other prior art devices, it does not possess high sensitivity, for two reasons. As noted above, the transducer is placed on a part of the body of the instrument, such as the soundboard, and hence is removed some distance from the point of string vibration and amplifies only the vibration sensed on the soundboard; and secondly, the piezoelectric crystals are not responsive to direct tension or compression but rather rely on compression as a result of the inertial mass of the transducer.